The present invention relates to a cross-linkable or cross-linked rubber composition usable in a tread of a tire, and a tire incorporating this tread. The invention applies in particular to tires of the passenger-vehicle type.
Since fuel economies and the need to preserve the environment have become priorities, it has become desirable to produce rubber compositions having good mechanical properties, low hysteresis for the manufacture of various semi-finished tire products, such as treads, and are useful in obtaining tires having reduced rolling resistance.
Numerous solutions have been proposed for reducing the hysteresis of tread compositions and the rolling resistance of tires comprising such compositions. See for example, U.S. Pat. No. 4,550,142, U.S. Pat. No. 5,001,196, EP-A-299 074 and EP-A-447 066.
In addition to this reduction in the rolling resistance, it is also desirable to improve the grip on dry or damp ground of vehicles fitted with these tires.
Solutions proposed in the past for improving some types of grip performance, such as grip on snow-covered or icy ground, involved replacing all or part of the aromatic oils conventionally used in tire treads of passenger-vehicle type with other specific plasticizers. The major disadvantage of these aromatic oils is their exudation from the tread by compression during prolonged travel, which causes the migration towards mixes adjacent to the tread due to the volatility of the aromatic oils. This loss of aromatic oil for the tread results in compaction, i.e., a reduction in the average height of the tread patterns, and hardening of the tread. The permanent deformation of the tread produces an adverse effect on the grip performance.
The disadvantage described above is more pronounced with larger quantities of reinforcing filler in the tread composition due to the higher requirement for aromatic oil as plasticizer.
Plasticizers comprising trioctyl phosphate or an alkyl oleate have been suggested as alternative constituents.
U.S. Pat. No. 5,852,089 discloses the use of a plasticizer obtained by esterification of diglycerol or an alkene oxide adduct of a polyhydric alcohol with an unsaturated fatty acid in a tread composition based on natural rubber and filled with carbon black to improve the grip of the corresponding tire, in particular on icy ground.
U.S. Pat. No. 5,852,089 discloses an ester type plasticizer which is present in a quantity of 7 phr (in association with 10 phr of aromatic oil) or of 30 phr (without aromatic oil) in the tread composition, and which is formed of the product of the reaction of the following consitutents (phr: parts by weight per hundred parts of elastomer(s)):                diglycerol and oleic acid, or        diglycerol and linoleic acid, or        a propylene oxide adduct of glycerol and oleic acid, or        a propylene oxide adduct of trimethylolpropane and oleic acid, or        a propylene oxide adduct of trimethylolpropane and linoleic acid, or        a propylene oxide adduct of pentaerythritol and oleic acid.        
Japanese patent specification JP-A-01 215838 discloses the use of a plasticizer in a tread composition filled with carbon black comprising:                an aromatic oil in a quantity of 25 phr,        a wood oil in a quantity of 15 phr, which is based on glycerides of eleostearic acid and oleic acid, and        a rosin type resin in a quantity of 15 phr.        
U.S. Pat. No. 5,252,649 discloses the use of other specific plasticizers to improve the grip of the corresponding tires on icy or snow-covered ground. The plasticizers tested in the examples of embodiment described comprise the following:                plasticizing oils extracted from petroleum (of aromatic and optionally naphthenic type), in a total quantity of from 20 to 50 phr, and        plasticizing compounds not extracted from petroleum comprising, in a quantity of from 5 to 15 phr, a rapeseed oil or, in a quantity of 5 phr, a sunflower oil, and optionally comprising glycerol trioleate in a quantity of 5 phr.        
Furthermore, these compounds not extracted from petroleum are present in the plasticizer in a mass fraction which is less than 43%.
European patent specification EP-A-561 761 discloses the use of plasticizers also intended to improve the grip of the corresponding tires on icy or snow-covered ground. The plasticizers tested in the examples of embodiment described comprise the following:                plasticizing oils extracted from petroleum (of aromatic and optionally naphthenic type), in a total quantity of up to 50 phr, or        a plasticizing compound not extracted from petroleum comprising, in a quantity of from 25 to 30 phr, a specific rapeseed oil.        
This rapeseed oil is such that it is derived from fatty acids in which oleic acid is present in a mass fraction of the order of 53%.
European patent specification EP-A-677 548 discloses the use of plasticizers in tread compositions comprising a specific silica (silica formed of granules which is sold by Degussa under the name “VN3”) as the reinforcing filler, and optionally carbon black, which is also intended to improve the grip of the corresponding tires on icy or snow-covered ground. The plasticizers tested in the examples of embodiment described comprise the following:                plasticizing oils extracted from petroleum, in a total quantity of from 22.5 to 36 phr, and        a plasticizing compound not extracted from petroleum comprising a rapeseed oil in a quantity of from 5 to 7.5 phr, such as the one used in the previous document or triethylene glycol dihexanoate in a quantity of 5 phr.        
It will be noted that the tread compositions presented in each of the latter three documents are specifically designed to have, in the cross-linked state, glass transition temperatures (Tg) which are relatively low due to the vegetable oils or the triesters used. These low temperatures are required to obtain satisfactory grip performances on snow or on ice.
There is a need to develop a rubber composition usable as a tread for a tire which imparts a high level of grip performance on dry and damp ground in comparison to known compositions, without adversely affecting the rolling resistance.